Polyethylene resins are known for the production of pipes and fittings. Pipe resins require high stiffness (creep rupture strength), combined with a high resistance against slow crack growth as well as resistance to crack propagation yielding impact toughness. However, there is the need to improve the creep rupture strength of currently available pipe resins, keeping the resistance against slow crack growth and the rapid crack propagation at least at a constant level. This would permit an increase in the pressure rating of such pipes.
Polyethylene pipes are widely used as they are lightweight and can be easily assembled by fusion welding. Polyethylene pipes also have a good flexibility and impact resistance, and are corrosion free. Unless polyethylene pipes are reinforced, they are however limited in their hydrostatic resistance by the inherent low yield strength of polyethylene. It is generally accepted that the higher the density of the polyethylene, the higher will be the long-term hydrostatic strength. Pipe resins are known in the art which are referred to by the names “PE 80” and “PE 100”. These classifications are described in ISO 9080 and ISO 12162. Extrapolation according to ISO 9080 shows that they have an extrapolated 20° C./50 years stress at a lower prediction level (97.5% confidence level—“LPL”) of at least 8 MPa [PE 80] and 10 MPa [PE 100]. The term “pressure pipe” in this specification refers to a pipe having a pressure rating of PE 80 and above.
There is a need in the art for polyethylene pipe resins which exceed the above test requirements. Currently, for polyethylene the highest hydrostatic strength which can be tolerated based on an extrapolation of the hoop stress/lifetime relationship at a temperature of 20° C. for a period of 50 years is an LPL of 10 MPa. This corresponds to a PE 100 resin. The density of the current basic powder used in the production of a PE 100 compound is close to 0.950 g/cm3 (typically from 0.949 to 0.951 g/cm3). Such polyethylene resins containing conventional amounts of black pigments have densities from about 0.958 to 0.960 g/cm3. There is now a desire in the art to produce a resin which when transformed into the form of a pipe, is capable of withstanding an LPL stress of 12.5 MPa at a temperature of 20° C. for a period of 50 years. Using the current terminology in the art, such a resin would be known as a “PE 125 grade” resin. Currently no such resins are commercially available.
Certain bimodal polyethylene resins are known to have very good hydrostatic strength. For example, WO 02/34829 discloses a polyethylene resin comprising from 35 to 49 wt % of a first polyethylene fraction of high molecular weight and from 51 to 65 wt % of a second polyethylene fraction of low molecular weight, the first polyethylene fraction comprising a linear low density polyethylene having a density of up to 0.928 g/cm3, and an HLMI of less than 0.6 g/10 min and the second polyethylene fraction comprising a high density polyethylene having a density of at least 0.969 g/cm3 and an MI2 of greater than 100 g/10 min, and the polyethylene resin having a density of greater than 0.951 g/cm3 and an HLMI of from 1 to 100 g/10 min.